This invention relates to improvements in oil spill recovery devices, particularly those of the type employing an endless reticular foam belt as disclosed in co-pending application Ser. No. 327,007, filed Jan. 26, 1973. While the invention is not confined in its application to mobile skimmer vessels operable on open water, it is herein illustratively described in such application. However, it will be appreciated that certain modifications and changes therein with respect to details may be made without departing from the essential features involved.
In order for a low-resistance flow-through filter belt of the type described to operate most effectively at maximum recovery rate, water bearing oil must not only have free access to the belt but must readily pass through the belt's thickness in preference to flowing around the belt. With such a low-resistance flow-through belt, the percentage of open pore area to total area of the belt presented to the flow in any cross section is sufficiently large that there is a natural tendency for the water to flow freely through it if there is no external barrier and no internal plugging or clogging. During such passage of the water, entrained oil is attracted to and held by the myriad of belt fibers which it contacts in the process of passage. According to the preferred operating technique with such a belt, the fibers are permitted to accumulate oil to a small percentage such as 15% of the belt's overall volume before the exposure to flow is interrupted, the belt purged of the accumulations, and the process repeated. In that manner the belt remains a free flow-through filter body, can be moved rapidly through the water without forcing the water to by-pass it, and, by the repetitious frequent purging of oil and reentries of the belt into the flow, can over a given period of time, recover great quantities of oil while leaving the processed water clean.
In the case of an endless filter belt moving with its lower portion submerged in the water and its upper portion out of water to permit purging it of oil, the speed of the belt's motion in its endless path must be sufficient to limit the accumulation of oil to less than the adsorption capacity of the belt so that the belt does not fail to attract and hold freshly encountered oil in performing its desired continued oil recovery action. Thus, in a practical mobile skimmer vessel using such an endless filter belt system, the belt is driven at a fairly rapid speed in its circuit, such as four linear feet per second. At such relatively high speeds, however, two conditions can develop which impair the belt's effectiveness as a free flow-through filtration medium capable of processing large volumes of water. First a certain amount of air is carried down into the water by the downwardly moving back stretch of the belt. Some of this air remains trapped on or in the belt. Some is released as a curtain of bubbles as the belt rounds the submerged guide roller and starts back up the upper or front stretch. Both effects of the retained air present some obstacles to free flow-through of the water to be filtered.
Secondly the rapid upward motion of the belt's front stretch leaving the water causes the belt to carry with it substantial volumes of water. This water raised as a wall or sheet immediately in front of the belt's front stretch, forms what amounts to a dynamic shield or barrier to free flow-through of oncoming surface water.
These two problems, just identified, become particularly acute when attempting oil spill recovery with little or no relative forward motion of the skimmer vessel, or when operating the recovery system in a stationary installation when little ambient current flows so as to carry the oil with water by force of the current itself through the recovery belt.
The present invention is directed generally to method and apparatus improvements by which these difficulties are largely overcome in a practical manner not adding unduly to the cost or complexity of the equipment, and not decreasing its recovery effectiveness (i.e., its filtration efficiency to remove all or most of the oil from water processed). A more specific object is to devise a flow enhancement means associated with the filter belt in such a manner that it does not agitate the water nor disperse or emulsify the oil, nor otherwise interfere with the filtration or separation function.
Still another object of the invention is to devise an improved mobile oil skimmer vessel more effective at widely varying operating speeds than heretofore, ranging from standstill to the highest speeds of which the belt system can be operated while functioning effectively.
A related object is to provide an oil recovery belt system for stationary or mobile applications wherein, without impairing recovery efficiency, the belt speed may be varied over a wide range to suit volumetric recovery rate imposed by the degree of concentration of the oil and relative movement of the water in relation to the system.